Radiation imaging apparatuses using a planar detector (hereinafter referred to as a Flat Panel Detector (FPD)) made of a semiconductor material are known as imaging apparatuses used for a medical image diagnosis or a nondestructive inspection using radiation (X-ray). In, for example, a medical image diagnosis, such a radiation imaging apparatus can be used as a digital imaging apparatus for capturing a still image or a moving image.
Examples of an FPD include an integral-type sensor and a photon counting-type sensor. Integral-type sensors measure the total amount of charge generated by incidence of radiation. On the other hand, photon counting-type sensors distinguish the energy (wavelength) of incident radiation and count the number of detections of radiation for each of a plurality of energy levels. That is, since photon counting-type sensors have an energy resolution, they can improve a diagnostic capability as compared with integral-type sensors. However, since the number of incident radiation quanta is enormous, a high operation speed is needed for the count of these radiation quanta. It has been therefore difficult to realize a large-area FPD with a photon counting-type sensor.
PTL 1 discloses a radiation imaging apparatus that has an energy resolution by estimating, for each predetermined region, the number of radiation quanta and an average energy using information about an average image density and information about the variance of image density. Using the technique disclosed in PTL 1, a sensor having an energy resolution can be realized even if the operation speed of the sensor is lower than that of a photon counting-type sensor.